It is recognized in the art that in electrochemical processes, it may be undesirable for certain dissolved metal ions to contact one of the electrodes. For example, as discussed further in U.S. Pat. No. 3,634,213, various dissolved metal salts may undergo an undesirable oxidation-reduction reaction whereby the metal forms an insoluble compound, or destroys the electrodes. This problem is solved in the process taught in said patent, by utilizing a cationic permiselective membrane to separate the anode and cathode compartments, whereby the dissolved metal salt is isolated from the electrode at which the undesirable oxidation-reduction reaction would occur. The electrodes utilized in the patent have configurations well known in the art, and further the patent does not discuss nor provide a solution for the problems which occur when the dissolved metal salt or other charged species, capable of undergoing an oxidation-reduction reaction, is to be oxidized or reduced at an electrode having the same charge.
In an electrochemical process for regenerating chromium plating bathes, wherein dissolved chromium in the +3 oxidation state is oxidized to the +6 oxidation state by contact with an anode, it is desirable, for maximum efficiency, to prevent contact of the dissolved chromium with the cathode.
U.S. Pat. No. 3,616,364 teaches the use of an electrolyzing anode and an electrolyzing cathode, said cathode being in intimate contact with a material in a low hydrogen overvoltage state, and said electrolyzing anode having a surface area at least equal to the surface area of said electrolyzing cathode, to continuously oxidize trivalent chromium to hexavalent chromium. Said electrolyzing anode and cathode, may be separated from the plating anode and cathode by a porous partition.
The problem, although not specifically discussed, of contacting the positively charged chromium, i.e., (r.sup.+.sup.3) with the positive electrode, i.e., the anode, to oxidize the chromium to the +6 oxidation state is solved by utilizing an anode having a large surface area. It would be desirable to have an anode having a large surface area while minimizing its physical dimensions.
The process taught in this patent, does not utilize a compartmentalized anode and cathode compartment, as does the process of the instant invention. As further discussed, hereinbelow, in a chromic acid regeneration process it is desirable to isolate the cathode from the dissolved chromium, by use of a semipermeable membrane. The patent teaches the use of a porous partition, to form separate compartments, however, said partition is permeable to all dissolved species, and furthermore, both the anode and the cathode are located in the same compartment.
U.S. Pat. No. 3,481,851 teaches a process for reoxidizing used chromic acid metal treating solutions by oxidizing said solution into the anode compartment of an electrodialysis cell, comprising an anode and cathode separated by a cation permeable membrane. The membrane is described as a commercially available material having a high percentage of cationic ion-exchange material. However it is well known that many membrane materials do not hold up under contact with chromic acid solutions or other oxidizing environments. Furthermore there is no teaching relating to obtaining optimum contact of the dissolved chromium with the anode, instead it was recognized by the patentee that the trivalent chromium was more often in contact with the membrane and thus a significant amount moved across the membrane into the cathode chamber.
U.S. Pat. No. 3,511,765, teaches a process for oxidizing or reducing organic compounds in an electrochemical cell, wherein both electrodes are provided in a liquid permeable form. There is no teaching that the electrodes may be isolated from each other by a semipermeable membrane, thus as pointed out by patentees, the process of the patent is limited to reactions where the oxidation product does not react at the cathode, or the reduction product at the anode.